Disc drives are commonly used in workstations, personal computers, laptops and other computer systems to store large amounts of data that are readily available to a user. In general, a disc drive comprises a magnetic disc that is rotated by a spindle motor. The surface of the disc is divided into a series of data tracks. The data tracks are spaced radially from one another across a band having an inner diameter and an outer diameter. As should be understood, to maximize the amount of data that can be stored on a disc surface, the inner and outer diameters of the data track band should be as close as possible to the inner and outer diameters of the disc itself.
Each of the data tracks extends generally circumferentially around the disc and can store data in the form of magnetic transitions within the radial extent of the track on the disc surface. An interactive element, such as a magnetic transducer, is used to sense the magnetic transitions to read data, or an electromagnetic element to generate magnetic flux that causes a magnetic transition on the disc surface, to write data. The magnetic transducer includes a read/write gap that contains the active elements of the transducer at a position suitable for interaction with the magnetic surface of the disc.
As known in the art, the magnetic transducer is mounted by a head structure to a rotary actuator and is selectively positioned by the actuator over a preselected data track of the disc to either read data from or write data to the preselected data track of the disc, as the disc rotates below the transducer. The head structure includes a slider having an air bearing surface that causes the transducer to fly above the data tracks of the disc surface due to fluid currents caused by rotation of the disc. The air bearing surface of the slider has a leading edge and a trailing edge. Typically, in currently used heads, such as, e.g., Transverse Pressure Contour (TPC) heads, two spaced rails are arranged to extend longitudinally along the lateral sides of the air bearing surface, one adjacent each lateral side, from the leading edge to the trailing edge of the surface. The rails provide various pressure effects to cause head flying operation.
Thus, the transducer does not physically contact the disc surface during normal operation of the disc drive. The amount of distance that the transducer flies above the disc surface is referred to as the “fly height”. It is a design goal to maintain the fly height of the head at an even level regardless of the radial position of the head.
In modern disc drives, a relatively rigid or hard disc is used as the magnetic medium. The disc comprises a hard substrate such as aluminum. Layers of various materials are applied to the surface of the aluminum substrate by, e.g., a sputtering process to provide layers that are substantially smooth and flat. The surfaces obtained from the sputtering process are designed to facilitate an even fly height for the head. The layered materials include a layer of magnetic material to provide the recording medium for the magnetic transitions representing data.
Typically, the outer diameter of the substrate is sloped at the radial outer end of the disc shape. This is referred to as the roll-off of the disc. Thus, at the outer diameter of the disc, the disc surface is no longer flat and usable to sustain a stable fly height of the air bearing surface of the head. Indeed, the flying behavior of the air bearing surface can become unstable if the head moves too far into the roll-off region of the disc, which can result in contact between the head and the disc surface. Any contact between the head and the disc surface may result in damage to the disc or head, leading to early disc drive mechanical failure.
Accordingly, it is important to design the disc drive such that the outer diameter of the data track band is spaced suitably inward from any portion of the disc roll-off region where fly height degradation can occur when reading data from or writing data to data tracks arranged at the outer diameter of the data track band. However, it is desirable that each disc used in a disc drive have a maximum radius relevant to the roll-off region that is equal to or greater than a preselected threshold radius so as to not impact the radial extent of the data track band beyond an acceptable amount.
To that end, during the manufacture of magnetic discs that are to be used in a disc drive, a check should be made of the roll-off radius of each disc as it moves through the manufacturing process, so as to reject any disc having a roll-off radius less than the preselected threshold value. In this manner, each disc made available for assembly into a disc drive will be able to accommodate a maximum data track band width for a maximized data capacity for the drive, without undesirable fly height instability or lack of clearance in the separation of recording head and media at the data tracks near the outer diameter of the data track band.
In a hard disc drive, certain clearance in the separation of recording head and media is required to avoid contacts and unstable flight at high rotation speed. Glide avalanche test is the common methodology used in the disc drive industry to monitor the spacing clearance U.S. Pat. No. 5,410,439. Outer Diameter Glide Avalanche (OD GA) has become an important gauge in qualifying media in recent years as a result of the requirements for low glide avalanche and high utilization of the disc surface. Performance of OD GA is affected by the disc edge roll-off. Thus, the problem of accurately measuring disc roll-off remains.
Conventional technique utilizes a contact profiler, such as Tencor P12, to measure disc topography in the radial direction near the edge of the disc. The dub-off and/or chord height computed from the measured trace is then reported. The dub-off value is defined as the maximum height undulation between two radii of the disc at the outer diameter. However, it has been determined that the dub-off value does not provide adequate information regarding fly height stability for a head positioned at a data track near or at the outer diameter of the data track band. In fact, there is a poor correlation between the dub-off value and fly height performance. Accordingly, the presently known disc measurement procedures do not provide an adequate system or process for achieving a reliable quality control for discs relevant to maximizing data capacity by assuring compliance by each disc with a maximum data band width having fly height stability at the outer diameter of the band.
A method using slope difference between lateral distances of a head width was suggested to improve the accuracy U.S. Pat. No. 5,497,085. However, the algorithm in that patent is still based upon displacement measurements on limited locations, which suffers in both sensitivity and variation of the measurement. The choice of differentiating slopes in the distance of head width limits the lateral resolution of this method, which cannot provide an effective inspection of the maximum available radius in meeting the disc specifications.